(d)
Since, $G$ is the centroid of a triangle, then
$\overrightarrow{\mathbf{GA}} + \overrightarrow{\mathbf{GB}} + \overrightarrow{\mathbf{GC}} = \overrightarrow{\mathbf{0}}\ \Longrightarrow \overrightarrow{\mathbf{GA}} + \overrightarrow{\mathbf{GC}} = - \overrightarrow{\mathbf{GB}}\ldots.(i)$
Now, $\overrightarrow{\mathbf{GA}} + \overrightarrow{\mathbf{BG}} + \overrightarrow{\mathbf{GC}} = - \overrightarrow{\mathbf{GB}} + \overrightarrow{\mathbf{BG}} = 2\overrightarrow{\mathbf{BG}}$
[ from Eq. (i)]